1. Field of the Invention
The present invention relates to an optical disc apparatus that controls a lens driving mechanism for driving an aberration correction lens in an optical axis direction according to a thickness of a cover layer of an optical disc.
2. Description of the Related Art
As an optical disc, a so-called multilayer optical disc is known such as Blu-ray Disc and DVD (Digital Versatile Disc), in which a recording layer is formed in a multilayered form. In such a multilayered disc, spherical aberration occurs in laser light converging to a recording layer due to a difference in thickness of a cover layer from a laser-light incident face to the recording layer among recording layers. Thus, especially in a multilayer optical disc such as Blu-ray Disc having improved recording density, the spherical aberration causes a failure in recording and reproducing, and thus, a lens driving mechanism is provided that is capable of driving an aberration correction lens in an optical axis direction according to the thickness of the cover layer of the optical disc in an optical pickup supporting the multilayer optical disc.
An optical disc apparatus including an optical pickup supporting a multilayered optical disc controls the lens driving mechanism so that the aberration correction lens is displaced in the optical axis direction corresponding to a recording layer for which recording and reproducing for the multilayer disc is performed.
The lens driving mechanism employs a stepping motor as a driving source for the aberration correction lens in general, and is configured such that a driving pulse for driving the stepping motor is generated from a motor driving circuit included in the optical disc apparatus so that the aberration correction lens is displaced in the optical axis direction. Also, lens control means for controlling the motor driving circuit is included in the optical disc apparatus, and the motor driving circuit is controlled by the lens control means to generate the driving pulse for driving the stepping motor.
The optical disc apparatus needs to grasp a displacement position of the aberration correction lens in order to control the aberration correction lens of the lens driving mechanism. The optical pickup includes a position detection element such as a photo interrupter as shown in Japanese Patent Laid-Open Publication No. 2007-328875, for example, so as to be capable of detecting that the aberration correction lens is displaced to a reference position, and measures a distance from the reference position, to control the displacement position of the aberration correction lens.
As mentioned above, if the optical pickup is configured including a position detection element for detecting a reference position of the aberration correction lens, the position of the aberration correction lens can be controlled based on the reference position.
However, provision of the position detection element in the lens driving mechanism involves an increase in the number of components and of assembling processes, which is disadvantageous in terms of cost. Thus, a lens driving mechanism without the position detection element is required, however, if there is no position detection element, it is necessary that a reference position of the aberration correction lens is set to one of mechanical movable limits of the aberration correction lens in the lens driving mechanism and that the aberration correction lens is reliably pressed onto the one of the mechanical movable limits of the lens driving mechanism to be displaced to the reference position.
If the aberration correction lens is pressed onto the mechanical movable limit of the lens driving mechanism, a mechanical load is applied to the lens driving mechanism, which causes deterioration in durability of the lens driving mechanism. The mechanical load of the lens driving mechanism is in proportion to a length of a time period during which the aberration correction lens is pressed onto the mechanical movable limit of the lens driving mechanism and to magnitude in kinetic energy of the pressing. Thus, when the aberration correction lens is returned to the reference position, it is necessary to reduce the length of the time period during which the aberration correction lens is pressed onto the mechanical movable limit of the lens driving mechanism as much as possible and to reduce the kinetic energy of the pressing as much as possible.